Same codename as Windows 2000. The codename was the key to activating an easter egg in Windows 98:

open the "Date and Time" control panel;

go to the "Timezone" page;

hold the Control key and drag a line with the mouse cursor from Memphis, Egypt (or maybe Cairo, the codename of Windows NT 4 - the map is too small to tell) to Memphis, Tennessee. Still holding the Control key, drag another line from Memphis to Redmond, Washington;

Microsoft originally planned for NT 4.0 to be just a simple "Shell Update Release" to integrate the Windows 96 GUI with Windows 3.51's kernel.

Wolfpack

Microsoft Cluster Server

Hydra

Terminal Services, Terminal Server

Windows NT 4.0 Terminal Server

Terminal Server adds "multithreading" support to Windows (the ability to run multiple instances of the graphics subsystem), and the Lernaean Hydra is a mythological monster with multiple heads.

Impala

Windows NT 4.1

Windows NT 4.0 Embedded

NT 5.0

Windows NT 5.0

Memphis

Windows 5, Windows NT 4.9

Windows 2000

Same codename as Windows 98. Its server counterpart is called Windows Server 2000.

Asteroid

Windows NT 5.0 Service Pack 1, Windows 2000 Service Pack 1

Janus

Windows NT 5.0 64-bit, Windows 2000 64-bit

Same codename as Windows 3.1

Neptune

Windows 2001, Windows. NET 2001

N/A

Ideas project, merged to Whistler.

Whistler

N/A

Windows Whistler

Merge of Whistler build 2202, Neptune and Project Odyssey. Named after Whistler, British Columbia, where design retreats were held.

Odyssey

Windows 5.1, Windows NT 5.1

Windows XP

Ideas project, merged to Whistler.

Sundown

Windows XP Professional x64 Edition

Mira

Windows XP Tablet PC Edition

Lonestar

Windows XP Tablet PC Edition 2005

Triton

N/A

Windows 2003

Mantis

Windows XP Embedded

Creekside

Windows XP Starter

Trident

Windows XP Media Center Edition

eHome

Windows XP Media Center Edition 2002

Freestyle

Windows XP Media Center Edition 2003

Name of a terrain rating system in the Whistler-Blackcomb resort.

Harmony

Windows XP Media Center Edition 2004

Name of a ski lift and alpine bowl in Whistler, British Columbia.

Symphony

Windows XP Media Center Edition 2005

Name of an alpine bowl in Whistler, British Columbia.

Emerald

Windows XP Media Center Edition 2006

Name of a ski lift in Whistler, British Columbia.

Trainyard

Windows XP Service Pack 1

Trainyard was an engineering package of driver updates to ship simultaneously with Windows XP Service Pack 1, the most major of which was support for USB 2.0 which was also ported backwards to Windows 2000.

Springboard

Windows XP Service Pack 2

Name of a ski run on Blackcomb in Whistler, British Columbia.

Whistler Server

N/A

Windows Server 2000 R2

Odyssey Server

Windows XP Server, Windows .NET Server

Windows Server 2001

One of many unrelated Microsoft projects of the time to carry a ".NET" designation. See: Microsoft .NET.

Triton Server

Windows 2003 Server

Windows Server 2003

Bobcat

Windows Small Business Server 2003

Bobcat is a ski-run on Whistler Mountain.

Longhorn Server

Windows Server 2005 (post-reset)

Windows Server 2004 (pre-reset), Windows Server 2003 R2 (post-reset)

Eiger

Windows Fundamentals for Legacy PCs

Software Assurance program. The Eiger is a mountain in the Bernese Oberland in Switzerland. The Eiger is part of a trio of mountains, The Eiger, The Mönch and The Jungfrau.

Mönch

Windows Mobile 6

VPN-s and advanced IP (Internet Protocol) security. The Mönch is a mountain in the Bernese Oberland in Switzerland. The Mönch is part of a trio of mountains, The Eiger, The Mönch and The Jungfrau.

Longhorn

Windows Longhorn

Named after the Longhorn Bar in the Whistler-Blackcomb resort; initially planned as an "interim release" between "Whistler" and "Blackcomb" (which is now "Vienna").

Six

Windows 6

Windows Vista

Went gold (RTM) on 8 of November 2006.

Cougar

Windows Vienna Small Business Server

Windows Small Business Server 2008

Same codename as Windows for Workgroups 3.2. Cougar is a ski-run on Whistler Mountain.

Server 6, Vista Server

Windows Server 2007

Home Server 6, Vista Home Server, Q, Quattro

Windows Home Server

Announced at CES 2007. Original project codename was "Quattro" because it was the leader of the project's 4th attempt at building a home server at Microsoft. Renamed to Q once the project moved out of incubation stage.

Vienna Server

Windows Server 2007 R2

Windows Server 2008

In development. Planned server equivalent of Windows Vienna to be released in 2008.

Blackcomb, Vienna

Windows Vienna

Originally codenamed "Blackcomb" after the Whistler-Blackcomb resort. Renamed to "Vienna" in January 2006.